Oil and process of manufacture of blended hydrorefined oil

ABSTRACT

A hydrocarbon fluid, useful as a textile lubricant or a coolant for a nuclear reactor or an electrical insulation medium (e.g., transformer oil) can be prepared from a severely hydrorefined oil having a viscosity from 30 to 500 SUS at 100*F. and an ultraviolet absorptivity of less than 0.04 in the 335 millimicron region by a process comprising increasing the content in the hydrorefined oil of polynuclear hydrocarbons having at least four condensed aromatic rings. When the severely hydrorefined oil has a VGC in the range of 0.82-0.92, the preferred 335 absorptivity of the blended product is in the range of 0.04 to 2.5. One preferred means of increasing the content of such polynuclear hydrocarbons having at least four condensed rings is to add to the severely hydrorefined oil a second oil (e.g., recycle stock from catalytic cracking of gas oil) which is rich in such polynuclear hydrocarbons.

States atent [1 1 Mills et al.

[ OIL AND PROCESS OF MANUFACTURE OF BLENDED HYDROREFINED OIL [75] Inventors: Ivor W. Mills, Media; Glenn R.

Dimeler, West Chester, both of Pa.

[73] Assignee: Sun Oil Company of Pennsylvania, Philadelphia, Pa.

[22] Filed: Dec. 10, 1973 [21] Appl. No.: 423,362

Related US. Application Data [63] Continuation .of Ser. No. 873,008, Oct. 31, 1969,

abandoned.

[52] US. Cl 208/14; 208/19 [51] Int. Cl. Cl0g 31/00 [58] Field of Search 208/14, 18, 19

[56] References Cited UNITED STATES PATENTS 2,846,372 8/1958 Schneider et a1, 208/14 3,095,366 6/1963 Schieman 208/14 3,252,887 5/1966 Rizzuti 208/14 3,318,799 5/1967 Acker et al. 208/14 3,369,999 2/1968 Donaldson et al 208/264 Dec. 23, 1975 Primary ExaminerHerbert Levine Attorney, Agent, or Firm-George L. Church; J. Edward Hess; Barry A. Bisson [57] ABSTRACT A hydrocarbon fluid, useful as a textile lubricant or a coolant for a nuclear reactor or an electrical insulation medium (e.g., transformer oil) can be prepared from a severely hydrorefined oil having a viscosity from 30 to 500 SUS at 100F. and an ultraviolet absorptivity of less than 0.04 in the 335 millimicron region by a process comprising increasing the content in the hydrorefined oil of polynuclear hydrocarbons having at least four condensed aromatic rings. When the severely hydrorefined oil has a VGC in the range of 0.82-0.92, the preferred 335 absorptivity of the blended product is in the range of 0.04 to 2.5. One

preferred means of increasing the content of such polynuclear hydrocarbons having at least four condensed rings is to add to the severely hydrorefined oil a second oil (e.g., recycle stock from catalytic cracking of gas oil) which is rich in such polynuclear hydrocarbons.

10 Claims, 6 Drawing Figures LLTRAVIOLET ABSORPTIVITY OF TRANSFORMER OILS "000-4005 I 'IIIII u a haunt-8 BI .09 DO .07 no I CURVE IDENTIFICATIGI n5 l GZS'F SEVERELYHYDROREFINEDOlL.

Z SULFUR": AGO REFINED OIL.

D4 3 Cl ACCORDING TO THIS INVENTION 4 MW MAILABLE HYOROREFHED OIL.

mm. MILLIWS US. Patent Dec. 23, 1975 Sheet1of6 3,928,168

ULTRAVIOLET ABSORPTIVITY OF TRANSFORMER OILS 5 FIGURE 1 .o: .09 .os '8; CURVE IDENTIFICATION 1 625! SEVERELY HYDROREFINED on z SULFURIC ACID REFINED OIL. o4 3 OIL ACCORDING TO THISINVENTION 4 COMMERCIALLY AVAILABLE HYDROREFINED on.

WAVELENGTH, MILLIMlCRONS US. Patent Dec; 23, 1975 sheet 2 of6 FIGURE 2 ULTRAVIOLET ABSORPTIVITY AT 335 m FOR 55 sus AT |oo F"ACID FREE NAPTHENIC DISTILLATE HYDROREFIN 0 AT VARIOUS TEMPERATURES s35 uvA-ase -e3 n5 so ass uvA o.|s

TOTAL L GEL- Q3 AROMATICS 30 I) n a i t 2 u- 0 & o a D v UVA AT 335 .0 rn o HYDROREFINING TEMPERATURE F (0.5 FEED LHSV, 4.0 RECYCLE LHSV; I200 PSIG OF 80 I. H2)

CATALYST! SULFIDED Ni M0 OXiDES ON ALUMINA TOTAL AROMATIC (ASTM D2057) US. Patent Dec. 23, 1975 NlTROGEN AND SULFUR REMOVED Sheet 3 of 6 FKEURE 3 SEVERE 450 500 550 600 MILD INCREASING REACTION SEVERlTY-- (0.5 FEED LHSV, I200 PSIG 0F 80% H2)4.0 RECYCLE LHSV US. Patent Dec.23, 1975 Sheet4of6 3,928,168

FIGURE 4 N=4O ppm N n: 4 N ppm 0 n.

k 2 pp 0 2O 4O 6O 80 I60 lO HOURS ON TEST (95C WITH AIR,COPPER) US. Patent Dec. 23, 1975 Sheet5 0f6 3,928,168

FIGURE 5 l I l 0 0.2 0.4 0.6 0.8 L0 L2 POLYNUCLEAR AROMATICS WHICH CONTRIBUTE TO 335 my. UVA

TOTAL ACID NUMBER AFTER 68 HRS. DOBLE OXIDATION TEST CONDITIONS US. Patent Dec. 23, 1975 Sheet 6 of6 3,928,168

FIGURE 6 TOTAL AROMATICS BY GEL IO 20 3O 4O 50 5 G O O IMPULSE BREAKDOWN STRENGTH, KV

OXIDATION STABILITY, HRS. ASTM D 943 6 0 d5 lb I5 2b 25 3.0'

% POLYNUCLEAR AROMATICS WHICH CONTRIBUTE TO 335 UVA CROSS REFERENCE TO RELATED A number of processes are known to the art for preparing such conventional transformer oils from distillate fractions of naphthenic crude oils. In general, such processes effect the removal of a substantial proportion of the naturally occurring naphthenic acids andthe sulfurand nitrogen-containing compounds which are present as impurities in the naphthenic distillate. A preferred means of removing the naphthenic acid im- APPLICATIONS Serial No. Filing Date Title 622,398 3-13-67 Clay Treatment of Hydrorefined Oils" Ivor W. Mills, Glenn R. Dimeler 636,493 5-5-67 Process for Preparing an Aromatic now Pat. No. Oil and Non-Discoloring Rubber 3,681,279 Composition Containing Said Oil Ivor W. Mills, Glenn R. Dimeler, Merritt C. Kirk, Jr.

652,026 7-10-67 Process for Producing Cable Oils now Pat. No. by Sequential Refining Steps 3,502,567 Ivor W. Mills, Glenn R. Dimeler 730,999 5-22-68 Hydrorefined Transformer Oil and Process of Manufacture" Ivor W. Mills, Glenn R. Dimeler 812,516 2-19-69 Catalytic Hydrofinishing of now Pat. No. Petroleum Distillates in the 3,619,414 Lubricating Oil Boiling Range Ivor W. Mills, Glenn R. Dimeler, Merritt C. Kirk, Jr., Albert T. Olenzak 850,779 8-18-69 Electrical Conduit Containing I-Iydrorefined Oil" Ivor W. Mills, Glenn R. Dimeler, John J. Melchiore 850,778 8-18-69 Process for Preparing High Visnow Pat. No. cosity Hydrorefined Cable Oil" 3,654,127 Ivor W. Mills, Glenn R. Dimeler,

William A. Atkinson, Jr., James P. Hoffman 850,716 8-18-69 Blended Hydrocarbon Oil and now abandoned Process of Manufacture Ivor W. Mills, Glenn R. Dimeler 850,717 8-18-69 Hydrorefined Lube Oil and Process now abandoned of Manufacture" Ivor W. Mills,

Glenn R. Dimeler 874,087 10-27-69 Light-Colored Highly Aromatic Oil and Process of Preparation" Ivor W. Mills, Glenn R. Dimeler and Merritt C. Kirk, Jr.

Now US. Pat. No. 3,462,358, issued August 19, 1969 The disclosure of all of the above-referred to applications (all of which are assigned to the Sun Oil Company) is hereby incorporated herein by reference, particularly as to disclosure therein directed to hydrorefined oils in the lube viscosity range (especially oils of -500 SUS at 100F.), to uses of such oils, to the production of such oils and to means of increasing the 335 UVA of a hydrorefmed oil.

BACKGROUND OF THE INVENTION The properties of commercial oils used as insulating media in transformers are well known in the art and a list of typical characteristics is given in the text by F. M. Clark entitled INSULATING MATERIAL FOR DE- SIGN AND ENGINEERING PRACTICE (1962), Page 135. Such oils typically boil in the range of 460775F. and have viscosities in the range of to 65 SUS (preferably to SUS) at 100F., as may be seen by reference to Wasson et al., U.S. Pat. No. 3,000,807 and to Wynkoop and Bartlett, U.S. Pat. No. 3,406,11 1. Preferably, the viscosity gravity constant of the transformer oil is in the range of 0.84 to 0.92.

purities from the distillate is shown, for example, in U.S. Pat. Nos. 2,770,580; 2,795,532; 2,944,014; 2,966,456; and 3,080,312, and can involve conducting the distillation in the presence of a base, such as caustic.

A less preferred method is to remove the naphthenic acid impurity at a later stage in the refining process, such as by contacting with a basic clay inthe final finishing or, in the case of hydrorefining, by converting the naphthenic acids to water and the corresponding naphthene (e.g., see THE MANUFACTURE OF ELECTRICAL INSULATING OILS by Jennings, H. C.

and Lawley, J. R., paper presented to the Edison Elecstage, they are reduced to naphthenes which contain alkyl groups, such as a methyl group, which are susceptible to oxidation to aldehydes or to acids during norinteraction of such compounds with a reagent, such as hydrogen, which can cause them to decompose to H or HN and the corresponding hydrocarbon, or involve interaction with a precipitant, such as an acid (e.g., H 80 HF, BF etc.) or involve the use of an adsorbent such as attapulgite and/or an acid-activated clay, or an acidic, crystalline aluminosilicate zeolite absorbent.

In such conventional processes for refining naphthenic distillate to produce a transformer oil, the process conditions are usually such that the removal of the sulfurand nitrogen-containing impurities is not selective and the aromatic hydrocarbon content of the distillate is altered by the treatment. In the case of hydrorefining (particularly severe hydrorefining at pressures above about 800 psi. of hydrogen and temperatures above about 500C., in the presence of catalysts containing cobalt, nickel, molybdenum, tungsten etc.), the total gel aromatic content (ASTM Test D2057) is substantially reduced. Of these aromatic compounds which are altered, the more highly condensed aromatics (such as the tetracyclic and pentacyclic condensed aromatic rings) are altered to a greater extent than are the less highly condensed aromatic compounds, such as those aromatic compounds which are polynuclear and contain only one aromatic ring.

The conversion by severe hydrorefming (under conditions favoring hydrogenation rather than dehydrogenation) of a polynuclear hydrocarbon having at least four condensed aromatic rings to a polynuclear hydrocarbon containing only one aromatic ring is illustrated by the following (reversible) chemical equations:

1n the present disclosure, a polynuclear hydrocarbon having four condensed aromatic rings will sometimes be referred to herein as a tetracyclic aromatic hydrocarbon. Similarly, polynuclear hydrocarbons containing one, two or three aromatic rings will be referred to, respectively, as monocyclic, dicyclic and tricyclic aromatics. Note that this nomenclature does not indicate the number of non-aromatic rings in the compounds denoted. Although non-aromatic rings influence the boiling range and fluid properties of the oil, we have discovered that the primary indicator of the oxidation stability (and impulse breakdown strength) of a transformer oil is the aromatic portion of the hydrocarbon, in particular, those aromatic substituents or groupings which contribute to the 335, 330 and 325 UVA of the oil.

Similarly, in the conventional acid treatment of naph- -thenic distillate to prepare transformer oil the conditions are such that (in order to obtain the desired deof the degree of removal of aromatic compounds from naphthenic oils by conventional acid treatment is found in copending US. Pat. application, Ser. No. 657,438 of Schneider and Stuart, filed May 29, 1967.

We have discovered that in transformer oils produced from a naphthenic distillate by severe hydrorefining, or by conventional acid refining (including acid refining of a raffinate from extraction with an aromatic selective solvent), the content of desired tetracyclic and higher polynuclear aromatic hydrocarbons is such that the oil has an ultraviolet absorptivity at 335 millimicrons of less than 0.04 and that the proportion of such desired polycyclic aromatic compounds which contribute to the 335 UVA compared with less desirable compounds which contribute to the ultraviolet absorptivity at 330 UVA is such that the ratio 335 UVA/330 UVA is less than 1.0. In such oils, the ratio 335 UVA/325 UVA is also less than 1.0 and commonly is less than 0.5.

For purposes of the present invention, the term 335 UVA denotes 335mMi/mp,(or the 334-336 mM region). Similarly, 330 UVA denotes 330i/mM (or the 329-331 mM region).

Polynucleer Hydrocarbon Having Four Condensed Aromatic Rings (i.e., Tetracyclic Aromatic Hydrocarbon) Monocyclic Aromatic Hydrocarbon Tricyclic Aromatic Hydrocarbon Dicyclic Aromatic Hydrocarbon SUMMARY OF THE INVENTION The present invention involves our discovery that the sludge-free Doble life (at a neutralization or total acid number below 0.2) can be greatly increased in a severely hydrorefined naphthenic distillate having a 335 UVA less than about 0.04 if the content in the oil "of polynuclear hydrocarbon having four or more condensed aromatic rings is increased such that the resulting oil has a 335 UVA in the range of 0.04 to 2.5, and, more preferably, in the range of 0.06 to 1.0.

We have further discovered that a particularly desirable, novel transformer oil with a Doble life of at least 100 hours and typically at least 200 hours (to as much as 300 hours) has a proportion of desirable tetracyclic and higher polynuclear aromatic hydrocarbons which contribute to the 335 UVA to those less desirable materials which contribute to the 330 UVA such that the ratio of the 335 UVA to the 330 UVA is less than 1.0. Preferably, when the oil contains less than p.p.m. of nitrogen and from 25 to 400 p.p.m. of sulfur, the 335 UVA is in the range of 0.06 to 1.0 (most preferably 0.15 to 0.4). Broadly, in the range of 25 to 800 p.p.m. of sulfur, the 335 UVA can be in the range of 0.06 to 2.0, although oils having a Doble life of 2 to 300 hours can be prepared having a 335 UVA in the range of 0.3 to 1.0. In such novel oils, the ratio 335 UVA to 325 UVA is commonly about 1.0 (e.g., 0.8 to 1.2).

One means of preparing such a novel transformer oil having a sludge-free Doble life greater than 100 hours is to add a highly dealkylated, highly condensed aromatic hydrocarbon to a severely hydrorefined naphthenic distillate having a 335 UVA less than 0.04. An especially preferred source of such highly dealkylated, highly condensed aromatic hydrocarbons is the recycle (boiling mainly in the range of 600l000F. at a 1 atmosphere) from the catalytic cracking of gas oil, particularly a gas oil which is cracked in the presence of a cracking catalyst containing a crystalline aluminosilicate zeolite cracking component (see, for example, U.S. Pat. No. 3,312,615). For preparing our novel transformer oil, distillate fractions of such catalytically aromatized and dealkylated oils (particularly said recycle stock), boiling mainly in the range of 650-800F. (more preferably 650-750F. are especially desirable components since their composition is such that the ratio 335 UVA/330 UVA is greater than 1.0 The 650750F. distillate is also preferred since it will no decrease the flash point or impart a brown or orange visible color to the oil. In general, our novel oils will have a light green fluorescent color and bloom.

Another means of improving the sludge-free Doble life of a severely refined naphthenic distillate having a 335 UVA less than 0.04 is to contact the hydrorefined oil in the presence of a dehydrogenation catalyst (such as sulfided nickel and molybdenum oxides on alumina) under conditions such that the tetracyclic and higher naphthenes and the polynuclear aromatics containing one, two or three aromatic rings in the hydrorefined oil are converted to tetracyclic and higher aromatic compounds, such catalytic contact being conducted at a liquid hourly space velocity (Ll-ISV) of the fresh feed and/or at a recycle LI-ISV, or combination thereof,

such that the resulting aromatized oil has a 335 UVA in v the range of 0.04 to 2.5 and, more preferably, in the range of 0.06 to 1.0 (most preferably in the range of 0.15 to 0.4).

To insure proper electrical properties, the oils produced by our process should be contacted with an adsorbent, such as attapulgite, in the final finishing stage of their manufacture.

6 BRIEF DESCRIPTION OF THE DRAWINGS In the attached drawings, FIG. 1 illustrates the ultraviolet absorptivity curves which are obtained from transformer oils prepared by various prior art processes and compares them with out novel transformer oil. In FIG. 1, curve 1 illustrates the UVA curve obtained from a severely hydrorefined naphthenic acid-free,

naphthenic distillate which has been clay contacted in the present invention (as will be described hereinafter).

Curve 4 is that of a commercially available hydrorefined transformer oil which contains a broad distribution of types of aromatic hydrocarbons and which does -not possess the type and distribution of tetracyclic and higher aromatic hydrocarbons as taught in our invention and, thus, has a ratio 336 UVA/330 UVA less than 1.0. Note that when oil 3 and oil 4 are compared in the 260 to 330 millimicron region, the shape of the absorptivity curves is very similar, indicating that the materials present in each oil which contribute to absorptivity in this region, are quite similar in nature and proportions. In contrast, in the 330-336 millimicron region the difference in the absorptivity curves is startling and indicates a substantial difference in the composition of the two oils.

FIG. 2 illustrates the effect of the degree of severity of the hydrorefining conditions on the ultraviolet absorptivity at 335 millimicrons for a feed comprising a naphthenic acid-free, naphthenic distillate. In FIG. 2 the reaction variable which is utilized as an indication of the severity of the hydrorefining is the hydrogenation temperature (in F.). Note that the 335 UVA is at a minimum at about 600F. and increases at higher temperatures, indicating that dehydrogenation and aromatization are at the higher temperatures. Similarly, the total gel aromatic content of the oil is at a minimum at 600F. and increases at higher temperatures.

FIG. 3 shows that as the reaction severity increases (as measured by the reaction temperature) the degree of nitrogen and sulfur removal from the oil also increases until (at about 550F. under'the conditions in FIG. 3) over of the nitrogen and sulfur has been removed from the oil. Note that at sufficiently severe hydrorefining conditions essentially all of the nitrogen and sulfur impurities can be removed from the oil.

FIG. 4 shows that the nitrogen content of a naphthenic transformer oil is an indication of the performance of the oil when aged at C. with air in the presence of copper. Therefore, when FIGS. 2, 3 and 4 are considered in combination, they indicate that the reaction severity in hydrorefining should be such that the degree of nitrogen removal is maximized (e.g., with sulfided Ni-Mo catlayst a reaction temperature in the 'range of 550600F. at 0.5 LI-ISV of fresh feed, 4.0

former oils and, therefore, can be very useful as transformer oils where an inhibitor is permitted. However, when it is desired that the uninhibited oil have a long sludge-free life under the Doble test conditions (i.e.,

over 64 hours), we have discovered that the hydrorefining conditions should be sufficiently severe as to both lower the nitrogen in the oil to less than 25 ppm (preferably, less than 15 ppm) and also to increase the content of tetracyclic and higher aromatic compounds contributing to the 335 UVA in order that the 335 UVA is greater than 0.04, and preferably at least 0.06. Under the reaction conditions shown in FIG. 2, such an oil having a Doble life of at least 68 hours can be obtained when the reaction temperature is greater than 680F., and preferably in the range of 685-765F. If, the fresh feed LI-ISV is less than 0.5, somewhat lower temperatures can be used to achieve the same degree of reaction severity as shown in FIG. 2(e.g., an oil with a 335 UVA greater than 0.04 can be prepared at 670F. and 0.25 LSHV). Similarly, at a higher LSHV, a higher reaction temperature is required to obtain this degree of severity. We have found that when hydrotreating with sulfided catalysts prepared from nickel molybdenum oxides or cobalt molybdenum oxides or nickelcobalt-molybdenum oxides (on the usual carriers, such as alumina), the preferred liquid hourly space velocity of the fresh feed is no greater than 1.0 at hydrogen pressures in the range of 800 p.s.i to 1600 p.s.i. At higher space velocities kinetic rather than equilibrium conditions occur and the resulting oils tend to be deficient in the desirable polynuclear aromatic hydrocarbons.

FIG. 5 shows the beneficial effect of polynuclear aromatic compounds (which contribute to 335 UVA) on oil stability under Doble oxidation test conditions (here the total acid number after 68 hours of Doble test conditions is used as an indication of oil stability). Note that for an acid number less than about 0.5 the oil must contain at least 0.2% of such polynuclear hydrocarbons containing at least four condensed aromatic rings.

FIG. 6 contains three curves which show the effect of such polynuclear aromatic compounds on oxidation stability under ASTM D-943 test conditions and under the impulse breakdown strength test, a third curve shows the effect of total aromatic content of the oil on the impulse breakdown strength (IBS). Note that this curve is a straight vertical line indicating that aromatics per se do not adversely effect the IBS, but that 188 is highly influenced by the type of aromatic compounds in the oil.

FURTHER DESCRIPTION OF THE INVENTION In the aforementioned patent application, Ser. No. 652,026, reference is made to out discovery that in the production of transformer oils from a 4070 SUS (at 100F.) naphthenic distillate by hydrorefining, it is advantageous to choose conditions (e.g., 625F., 1200 p.s.i.g. of 80% H such that the sulfur and nitrogen contents of the distillate are substantially reduced and there is a concomitant partial saturation of polycyclic aromatic hydrocarbons such that the ultraviolet adsorptivity of the hyrorefined product at 335 millimicrons (335 UVA) is below 0.04 (preferably below 0.01).

Such severe hydrorefining can be at a temperature in the range of 500 to 750F. and from 800-3000 p.s.i. of hydrogen partial pressure at a liquid hourly space velocity of from 0.1-8.0, preferably conducted either in vapor phase or trickle phase. Product recycle, for example as in US. Pat. No. 2,900,433, can be used, preferably at a product to fresh feed ratio below :1 (more preferably 8:1 to 1:1). Preferably the temperature is below that at which substantial cracking occurs; that is, no more than 20% (preferably less than 10%) of the feed stock is converted to material boiling below ditions, we prefer to operate below 700F., more preferably below 675F. At total pressures below 2000 p.s.i.g. and with a fairly fresh catalyst, we prefer a temperature no greater than 650F. since above that temperature aromatization can occur and/or the produc- -tion of low boiling material and the degradation of oil viscosity can become substantial. After some months of use, if the activity of the catalyst decreases appreciably, a higher temperature (c.a. 675F.) can be used to prolong catalyst life, i.e., to delay regeneration or replacement of the catalyst.

Typical of such severe hydrorefining methods, which can be used in our process when conducted within the processing conditions referred to herein, are those of US. Pat. Nos. 2,968,614; 2,993,855; 3,012,963; 3,114,701; 3,144,404 and 3,278,420.

Typical catalysts are molybdenum oxide, cobaltmolybdenum oxides, nickel-molybdenum oxides, cobalt-nickel molybdenum oxides and tungsten-nickel molybdenum oxides, preferably presulfided and on a carrier such as silica, alumina, alumina-titania and alumino-silicates (either crystalline or amorphous). Nickel sulfide, nickel-molybdenum sulfide, tungsten disulfide, nickel-tungsten sulfide and molybdenum disulfide, per se or on a carrier, can also be used as catalysts. Examples of operable catalysts are those of US. Pat. Nos. 2,744,052; 2,758,957; 3,053,760; 3,182,016; 3,205,165; 3,227,646; and 3,264,211.

We prefer that such severe hydrorefining (when under conditions favoring hydrogenation) be a trickle phase process (although gas-phase operation with hydrogen recycle up to 12,000 SCF/B can be utilized) at 525675F. and 800 to 1600 p.s.i. of hydrogen partial pressure using a catalyst comprising nickel and molybdenum sulfides on alumina or silica. Usually a cobaltmolybdenum catalyst will require 25 to 100 percent greater hydrogen pressure, at a given temperature, recycle and LHSV, to produce a severly hydrorefined transformer oil comparable to that obtained with a sulfided nickel-molybdenum catalyst.

As has been noted in US. pat. No. 2,973,315, the severity of hydrogenation can be measured by the hydrogen consumption. With a 40 to 200 SUS (more usually 45 to SUS) naphthenic distillate feed, reaction severity can also be followed by observing the change in ultraviolet absorptivity, as in the 260 millimi- -cron region (hereinafter sometimes the 260 UVA). For

transformer oil manufacture according to the present invention, the change in absorptivity in the 335 millimicron region (335 UVA) should be observed. That is, due to hydrogenation of polycyclic aromatic hydrocarbons, the resulting hydrogenated oil will have lower ultraviolet absorptivity in the 335 millimicron region (and in the 260 mM region) than will the base oil before hydrogenation. FIG. 2 illustrates the effect of hy drorefining severity on 335 UVA.

Typically, after severe hydrogenation (under nonaromatizing conditions), the 260 mM absorptivity of the naphthenic distillate is reduced by from 40 to and is less than 5.0 for a 40 to 70 SUS naphthenic distillate feed (more commonly less than 3.5). Preferably, in the 40 to 70 SUS range, the hydrogenated oil will contain less than 0.1% sulfur and less than 25 p.p.m. of nitrogen.

Due to differences in aromatic, sulfur and nitrogen content of the base oils, hydrogen consumption can vary greatly; however, hydrogen consumption for severe hydrorefining of a naphthenic acid-free distillate is usually at least 150 SCF/bbl. For an indication of the large hydrogen consumption in severe hydrorefining (under non-aromatizing conditions) see US. Pat. No. 2,973,315 and the aforementioned US Pat. application, Ser. No. 622,398. One cause of high hydrogen consumption is that we prefer to hydrorefine at conditions (e.g., 575-650F., 900 to 1500 p.s.i.g. of H total pressure 800 to 3000 p.s.i.g., no gas recycle, sulfided Ni-Mo catalyst) such that the total gel aromatics in the feed to the hydrorefining step are reduced by about to (mainly due to removal of polar compounds) and most to of the dicyclic and higer aromatics in the feed are converted to monocyclic aromatics. In particular, under such conditions, the 335 UVA of the resulting severely hydrorefined naphthenic distillate will be less than 0.04.

In contrast, mild hydrogenation processes frequently consume less than SCF/ of H /bbl. and are characterized by little change in polycyclic aromatic content of the oil (in particular, the 335 UVA after hydrotreating is greater than 0.04 for 40 to 70 SUS naphthenic feed). Mild hydrogenation is frequently termed hydrotreating and is usually conducted below 800 p.s.i. of hydrogen or below 500F. Typical of mild hydrogenation treatments are US. Pat. Nos. 2,865,849; 2,921,025; 2,944,015; and 3,011,972.

Although impulse breakdown strength testing is routinely applied to finished transformers (ASA Standard C.57.l2.00-l965 there is a growing tendency to apply this type of testing directly to the transformer oil. A segment of the electrical industry believes that the oil impulse breakdown strength can influence the impulse breakdown strength of the oil impregnated coil and core structure of the finished transformer.

The test as applied to the oil, uses needle-sphere electrodes with a one inch gap. A l k X 40 negative wave is applied to the electrodes (as specified in paragraph 8.2.l of the above-mentioned ASA standard). The breakdown voltages of the oils reported herein are the average of five separate determinations.

Oxidation stability of a transformer oil is improved by reducing nitrogen and sulfur content and by retaining (or returning) polynuclear aromatic compounds which contribute to the 335 UVA and which act as natural inhibitors. Where, however, it is desired to have an impulse breakdown strength in a transformer oil of greater than about 120 KV, such polynuclear aromatics which contribute to the 335 UVA are not desirable. If the hydrotreatment conditions are adjusted such that the resulting hydrorefined oil has a 335 UVA less than about 0.04 the impulse breakdown strength of the resulting severely hydrorefined naphthenic distillate can be as high as KV, as is shown by the curve labelled IBS on the accompanying FIG. 6, which relates impulse breakdown strength to the polynuclear aromatic content of the hydrorefined oil. The percent polynuclear aromatics was determined by measuring the 335 UVA of the oil and comparing it with a calibration curve derived by measurement of the 335 UVA of samples containing known amounts of tetracyclic polynuclear aromatic compounds. In FIG. 6 (and also FIG. 5), a tetracyclic and higher polynuclear aromatic content of 1.0% corresponds to a 335 UVA of 0.15. The calibration curve is linear. At a UVA of 0.45, the polynuclear l 0 content is 3% and at a polynuclear content of 0.3% the 335 UVA is 0.045.

Also shown in FIG. 6, is a curve relating the oxidation stability as measured by ASTM Method D943 to the percent of polynuclear aromatics contributing to the 335 UVA. This curve is labelled D943 and, when taken in conjunction with the IBS curve on the same figure, shows that in an uninhibited transformer oil prepared from naphthenic distillate it is not possible to maximize both oxidation stability and impulse breakdown strength. However, within the limitations imposed by the laws of physics and chemistry, the curves in the figure can be used by the refiner to enable him to tailormake an oil having a particular combination of impulse breakdown strength and oxidation stability. That is, the refiner can adjust his refining process (either by choosing aromatizing or non-aromatizing hydrorefining conditions or by blending, as by the addition of polynuclear aromatics to a severely hydrorefined oil) so that the tetracyclic and higher polynuclear aromatic content of the oil (as measured by the 335 UVA) will be such that the desired properties are obtained.

Through the use of conventional oxidation inhibitors,

- such as 2,6-di-tertiary-butyl-p-cresol (DBPC), and the process of the present invention, severely refined transformer oils can be produced which have a maximum impulse breakdown strength and excellent oxidation stability.

In FIG. 6, the nearly vertical straight line represents the impulse breakdown strength as a function of total aromatic content of the oil and demonstrates that total aromatic content does not control impulse breakdown.

We have discovered that transformer oils having excellent color and oxidation resistence can be prepared from severely hydrorefined oils either by the addition thereto of polynuclear aromatic compounds contributing to the 335 UVA or by adjusting the hydrorefining conditions (in a second step or directly from the distillate in one step) such that sufficient hydroaromatization occurs as to convert a portion of the more highly saturated condensed polycyclic hydrocarbons in the oil to more highly aromatic hydrocarbons which contribute to the 335 UVa and which are effective as oxidation inhibitors. We have further discovered that the preferred hydrorefining conditions for severe hydrorefining (whether with or without aromatization) should include a liquid hourly space velocity (LHSV) for the fresh feed of no more than 1.0 and preferably no greater than 0.5. This is to be contrasted with the process of US. Pat. No. 3,252,887, which utilizes an LHSV in the range of l to 3, more typically about 2.0.

In general in the present invention, the hydroaromatization of the severely hydrorefined naphthenic distil-- late will require a temperature above 675F;, a hydrogen pressure below l600 p.s.i.(and preferably a total pressure below 2000 p.s.i.) and a fresh feed LHSV less than 2.0 (preferably less than 1.0). In addition to the when the sulfur content of the hydrorefined oil is sufficiently low as to not cause catalyst poisoning.

Our preferred conditions for severe hydrorefining wherein aromatization occurs involve a temperature above 625F. but below the temperature where substantial hydrocracking will occur, a hydrogen partial pressure in the range of 800 to 1600 p.s.i., a total pres- 1 1 sure in the range of 1000 to 2000 p.s.i.g. and an LHSV of fresh feed in the range of 0.1 to 1.0, in the presence of a dehydrogenation catalyst, such as a catalyst con-' taining nickel and molybdenum or compounds of these metals. Also operable, but not preferred, are an LHSV below 2.0 and a total pressure as low as 500 p.s.i.g.

The aforementioned U.S. Pat. No. 3,252,887, involves blending a high temperature hydrorefined distillate (hydrorefined at a temperature in the range of 630670F.) with a low temperature hydrorefined distillate (hydrorefined at a temperature in the range of 555595F.), wherein the hydrorefined oils are individually prepared by contacting a naphthenic distillate with a hydrorefining catalyst at an LIISV of 1 to 3 and a pressure of 500 to 900 p.s.i.g. We have discovered that highly desirable transformer oils can be prepared by blending from to 85 volume of a severely hydrorefined naphthenic distillate having a 335 UVA less than 0.04 (e.g., under the conditions of FIG. 2, the oil hydrorefined at 600F., and more generally, an oil hydrorefined in the range of 575-675F.) with from 85 to 15% of a severely hydrorefined oil having a high 335 UVA (e.g., 0.6 to 1.2) wherein the hydrorefining was conducted under conditions favoring aromatization (preferably, at a temperature in the range of 725800F., more preferably a temperature in the range of 760780F.) and wherein said hydrorefined oils have been individually prepared by contacting a naphthenic distillate (preferably, naphthenic acid free) with a hydrorefining catalyst at an LHSV of the fresh feed of 0.1 to 1.0, a total pressure of 1000 to 2000 p.s.i.g. and a hydrogen pressure in the range of 800 to 1600 p.s.i. The final blended oil, or the individual components thereof, should be contacted with an adsorbent clay, such as attapulgite.

As we have noted in our previously cited application,

Ser. No. 622,398, now U.S. Pat. No. 3,462,358 issued Aug. 19, 1969, it is sometimes advantageous in compounding hydrorefined electrical oils to blend the base oil with from 0.05 to volume percent of such additives as chelating agents, viscosity modifiers, synthetic paraffinic oils, hydrotreated oils, acid-treated naphthenic coils and napthenic distillate (particularly naphthenic distillate which is substantially free of naphthenic acids). Such blending can add to the flexibility of the present invention (see Ser. No. 850,716).

I-Iydrorefining as practiced in our invention can produce oils having product quality which is comparable to and in many cases exceeds that of prior art oils prepared by solvent extraction and/or acid treating. I-Iydrorefining when compared with acid treating and solvent extraction also possesses the advantages of allowing increased yields and eliminating low quality or troublesome by-products.

Another advantage of our invention is process flexibility in that our process can make a wide variety of products with differing properties from the same naphthenic distillate feed stock. This flexibility is apparent in the curves of FIG. 6, which teach the art how to obtain transformer oils having different values of impulse breakdown strength and oxidation stability.

ILLUSTRATIVE EXAMPLES In the following examples, Example I illustrates the severe hydrorefining of a naphthenic acid free, naphthenic distillate to produce a severely hydrorefmied transformer oil having a 335 UVA less than 0.04.

Example II shows the preparation of a novel, transformer oil by adding to the severely hydrorefined oil of Example I a minor proportion of a distillate fraction of the recycle stock from catalytic cracking of gasoline.

Example 111 shows the preparation of a transformer oil having a Doble life greater than 64 hours by hydrorefining a naphthenic distillate in a single stage under conditions such that the 335 UVA of the resulting hydrorefined oil is greater than 0.04 and the nitrogen no greater than ppm.

Example IV shows the catalytic aromatization of the severely hydrorefined oil of Example I under conditions such that a portion of a more saturated polynuclear hydrocarbon present in the oil is converted to polynuclear hydrocarbon which contributes to the absorptivity at 335 millimicrons, thereby producing a transformer oil having an increased sludge-free Doble life compared with that of Example 1.

Example V shows that hydrorefining conditions can be adjusted such that a transformer oil can be produced by hydrorefining which has a high impulse breakdown strength and a good performance in ASTM Oxidation Test D1313 but that the sludge-free Doble life of such an oil is not as high as has heretofore been considered desirable in a transformer oil.

Example VI demonstrates that the inhibitor response of transformer oils can differ greatly and that inhibitor response is not necessarily a function of the stability of the uninhibited oil.

EXAMPLE I A naphthenic acid free, naphthenic distillate having a 5% point of 564F. and a 95% point of 660F. was contacted with a catalyst prepared by sulfiding a composite of nickel and molybdenum oxides on alumina. The hydrorefining conditions were those shown in FIG. 2 hereof and included 0.5 feed LHSV, 4.0 recycle LHSV, 1200 p.s.i.g. of 80% H and a hydrorefining temperature of 625F. After contacting with 10 lbs/barrel of attapulgite, the resulting severely refined transformer oil had a sludge-free Doble life of less than 48 hours. The properties of the feed and product of Example I are shown in Table I hereof.

EXAMPLE II A recycle oil (from catalytic cracking of 450-750F. gas oil with a catalyst containing 10% of a rare earth exchanged Type Y zeolite in an amorphous aluminosilicate matrix) was distilled to obtain a fraction boili- 50 nig in the range of 650-750F. Table II hereof reports the ultraviolet absorptivities at 260, 325 and 335 millimicrons of distillate fractions of the recycle oil.

Two volumes of the 650750F. distillate fraction of the recycle oil were blended with 98 volumes of the 55 severely hydrorefined, adsorbent contacted product of Example I. The resulting blending oil had a 335 UVA of 0.35, and the ratio 335 UVA to 330 UVA was greater than 1.0. This blended oil product had an impulse breakdown strength of 110 and a sludge-free 60 Doble life of greater than 150 hours. The ultraviolet absorptivity curve of the resulting blended oil is shown in FIG. 1 and other properties of this oil are reported in Table I as Example II product. The use of such an unrefined distillate to improve the sludge-free Doble 65 life of a hydrorefined oil is to be contrasted with such prior art disclosures as that of U.S. Pat. No. 2,846,372, which teaches that subsequent refining of such a distillate, as with sulfuric acid and/or sodium hdyroxide, is essential for preparation of an oxidation inhibitor for non-hydrorefined oil.

EXAMPLE IIl Example I was repeated except that the hydrorefining temperature was 750F. and the feed LHSV 1.0 with no liquid recycle. The resulting oil had a 335 UVA of 0. l 6, contained 47.4% aromatics, and had a Doble life of 100 hours.

EXAMPLE V and a sludge-free Doble life of 48 hours. Properties of the transformer oil produced in this example are listed in Table I as Example IV product.

TABLE I Properties of Transformer Oils Example I Example I Example 11 Example V Conventional Conventional Feed Product Product Product Transformer* Transformer Viscosity, SUS/100F. 57.2 57.5 57.4 57.9 57.3 57.2 Gravity, AP1 25.1 26.5 26.0 26.5 25.7 25.9 Total Nitrogen, ppm 74 24 23 10 5 Total Sulfur, wt. 0.17 0.04 0.05 0.05 0.15 0.05 Aromatics, wt. 25.5 27.3 26.9 35 32 Performance Oxidation ASTM D943, hours, Uninhibited 80 I3 204 68 90 90 Oxidation ASTM D-1313, Sludge (GE Bomb) 0.14 76 0.05 0.06 0.060.08 0.06 Impulse Breakdown Strength, (KV) 170+ 110 162 152 335 UVA 0.09 0.015 0.35 0.024 0.045 0.019 325 UVA 0.38 0.05 0.35 0.083 0.25 0.17 260 UVA 5.1 1.2 3.8 1.48 3.5 2.5 Sludge-Free Doble Life (Hours) 24 48 150+ 48 48 64 "1O lb/bbl of 99% H250 of acid-free naphthenic distillate "35 lb/bbl of 99% H,SO treatment of furfural raffinate of acid-free naphthenic distillate TABLE II Boiling Range and Absorptivities of Recycle Oil from Catalytic Cracking of 450700F. Gas Oil Absorptivities Distilled Temp. F. at 760 mm at 260 my. at 325 mu at 335 mp.

Initial 372 TABLE III Inhibitor Response of l-lydrorefined Transformer Oils Polynuclear Aromatics Which D943 Oil Contribute to Life DBPC Required Num- 335 UVA No DBPC 500 hours 1000 hours 1500 hours her Oil No. l Prepared similarly to Example 11 Product but with 295% of the recycle from a different catalytic cracker.

Oil No. 2 prepared similarly to Example 1 Product but at hydrorefining temperature of 650'F. in order to generate 0.1% pulynuclear aromatic compounds.

Oil No. 3 prepared similarly to oil of Example V.

Oil No. 4 prepared similarly to Example 1 Product but at 600F.

EXAMPLE IV EXAMPLE VI The pr d o Examp I Was contacted in a Second 5 Although oxidation stability is normally specified by hydrorefining Stage with the Example I catalyst under the electrical industry for uninhibited oils, transformer conditions favoring aromatilahoh 'LHSV oils usually contain an inhibitor, such as DBPC, when 110 y 1200 P- 0f 80% y g The resulting incorporated in a transformer: The inhibitor response Oil had a UVA 0f ahd 3 Doble life of greater than of a number of oils was studied by determining the hours.

weight percent of DBPC required to allow the resulting inhibited oil to perform satisfactorily at ASTM D-943 test conditions for 500, 1000 and 1500 hours. Table III reports the result of these studies of inhibitor response and shows that inhibitor response is related to the concentration in the oil of polynuclear aromatic compounds which contribute to the 335 UVA.

The data in Table III demonstrate that all oils do not have a comparable inhibitor response and that inhibitor response is not necessarily a function of the stability of the uninhibited oil. Note that the oils containing the lower percent of polynuclear aromatic hydrocarbons tend to show a higher response to the additive. Oil No. 1 requires approximately 0.2% DBPC to attain 1000 hours life; whereas, Oil No. 4 containing no polynuclear aromatics requires only 0.1% DBPC to provide the same protection. This example shows that the oxidation stability of an oil in the absence of an inhibitor does not necessarily indicate the stability of the oil when inhibited with a given percent of an inhibitor.

98-60 parts by weight of oils such as those of Examples I, III, IV and V can be admixed with from 2-30 parts by weight of raw naphthenic distillate (preferably, free from naphthenic acids) to obtain an oil admixture having improved performance in such tests as ASTM D-13l3 and D-943.

For example, a blend of 1-40 parts by weight of raw naphthenic oil (or naphthenic acid free naphthenic oil), 0.1- parts by weight of cycle oil and from 97-50 parts by weight of hydrogenated naphthenic distillate can be useful as an electrical insulation medium. This unclaimed disclosure is related to the invention claimed in copending application Serial No. 850,716.

In a broad aspect of the present invention, a hydrocarbon fluid, useful as a textile lubricant or a coolant for a nuclear reactor or an electrical insulation medium (e.g., transformer oil) can be prepared from a severely hydrorefined oil having a viscosity from to 500 SUS at 100F. and an ultraviolet absorptivity of less than 0.04 in the 335 millimicron region by a process comprising increasing the content in the hydrorefined oil of polynuclear hydrocarbons having at least four condensed aromatic rings. When the hydrorefined oil has a 335 UVA less than about 0.02 and is very low in gel aromatics (such as a white oil, or spray oil made by hydrorefining a petroleum stock having a viscositygravity constant in the paraffinic range), a significant improvement in oxidation stability can be obtained by blending the hydrorefined oil with from about 0.5 to 25 volume of the unhydrorefined base stock from which the hydrorefined oil was obtained. Typical base stocks (preferably having a 335 UVA of at least 0.04) are hydrodesulfurized oils, paraffinic or naphthenic petroleum distillate oils (preferably pretreated to remove or reduce asphalt, was or naphthenic acids), lube oils produced by hydrocracking petroleum fractions, and the raffinate obtained by extracting such oils with an aromatic selective solvent (such as furfural or phenol).

For example, an improved textile oil was obtained by blending 90 parts by volume of a 60 SUS (at 100F.) spray oil produced by hydrorefming (see Ser. No. 850,717) a solvent dewaxed paraffinic distillate having a 335 UVA of 0.05 and 10 parts by volume of the unhydrorefined base stock (60 SUS solvent dewaxed paraffinic distillate). The hydrorefined spray oil had a 335 UVA of 0.010 and a total acid number after ASTM D972 testing of 3.14. The blend of hydrorefined and unhydrorefined oil had an acid number after the D972 testing of 0.41 and a 335 UVA of 0.020.

EXAMPLE VII Test Blended Oil Viscosity, cST at 20C. 20 Flash Point (closed), D-93, F. 305 Pour Point, F. 50 Total Acid No., mg KOH/g 0 Saponification No., mg KOl-I/g Nil Copper Strip, D-l30 IA (pass) Michie Oxidation Test (IP56) Sludge 0.85

Total Acid No. 0.90

It can be seen from the foregoing that an improtant step in our process can be the determination of the ultraviolet absorptivity of a hydrorefined oil for at least one wave length (or band) in the 320-340 millimicron region, preferably at about 335 mM. Such a measurement is useful for quality control and for determining how much of an oil rich in tetracyclic aromatics should be blended with the hydrorefined oil.

The unclaimed disclosure herein relating to novel transformer oils and to increasing the 335 UVA of a hydrorefined oil by catalytic aromatization of a more saturated polynuclear hydrocarbon present in the oil to produce a polynuclear hydrocarbon which contributes to the 335 UVA, is related to the invention claimed in said Ser. No. 730,999.

The unclaimed disclosure herein relating to contacting a lubricating oil distillate stock with hydrogen in the presence of a hydrogenating component selected from the group consisting of sulfides and oxides of (a) molybdenum and at least two iron group metals and (b) a combination of nickel and tungsten, said contacting being carried out at a temperature of about 500-800F., at a space velocity of about 0.1-8.0 liquid volumes of oil per volume of catalyst per hour based on the fresh feed, and at a pressure of about 500-3000 p.s.i. of hydrogen, the combination of conditions being so selected as to effect appreciable hydrogen consumption but no substantial cracking is related to the invention claimed in said Ser. No. 812,5l6.

The unclaimed disclosure herein relating to hydrorefined oils having a viscosity in the range of l00-l2,000 SUS at F. and containing l0-44 wt. gel aromatics and less than 10 ppm. of basic nitrogen is related to the invention claimed in said Ser. No. 850,717.

The invention claimed is:

1. Process for preparing a hydrocarbon fluid of improved oxidation stability from a severely hydrorefined oil having a viscosity from 30 to 500 SUS at 100F and an ultraviolet absorptivity of less than 0.04 in the 335 millimicron region, said process comprising adding to the severely hydrorefined oil an unrefined distillate of a recycle stock obtained from catalytic cracking of gas oil which is rich in polynuclear hydrocarbons having at least four condensed aromatic rings and recovering a blended oil having an absorptivity at 335 millimicrons at least 15% greater than that of said hydrorefined oil and less than 2.5.

2. Process according to claim 1 wherein said hydrorefined oil has a sludge-free Doble life of less than 48 hours and the resulting blended oil has a 335 UVA in the range of 0.4-2.5 and is contacted with sufficient amount of an adsorbent to produce a hydrocarbon oil, useful as an electrical insulator oil, and having a sludgefree Doble life of at least 64 hours.

3. Process according to claim 2 wherein said hydrocarbon oil product is useful as a transformer oil and has a sludge-free Doble life of at least 100 hours with a total acid number less than .1, said oil having a viscosity at 100F. in the range of 45-70 SUS, an ultraviolet absorptivity at 335 millimicrons in the range of 0.06-2.0, a sulfur content in the range of 25-800 p.p.m., and containing less than 25 p.p.m. of total nitrogen and less than 10 p.p.m. of basic nitrogen, the chemical composition of the hydrocarbons in said oil being such that the ratio of the ultraviolet absorptivity at 335 millimicrons to the absorptivity at 330 millimicrons is no less than 1.0.

4. A process according to claim 3 wherein said hydrocarbon oil has a sludge-free Doble life of at least 200 hours at a total acid number less than 0.01 and wherein said absorptivity at 335 millimicrons is in the range of 0.3 to 1.0.

5. Process according to claim 1 wherein the ultraviolet absorptivity of said severely hydrorefined oil is determined for at least one wave length or band in the 320-340 millimicron region.

6. Process according to claim 5 wherein said wave length or band is at about 335 millimicrons.

7. Process for preparing a hydrocarbon fluid useful as an electrical insulation medium from a severely hydrorefined naphthenic oil having a sludge-free Doble life less than 48 hours and a viscosity from 40 to 200 SUS at 100F and having an ultraviolet absorptivity of less than 0.04 in the 335 millimicron region, said process comprising adding to said hydrorefined oil an unrefined distillate of a recycle stock obtained from catalytic cracking of gas oil and, which is rich in polynuclear hydrocarbons having at least four condensed aromatic rings, until said abosrptivity at 335 millimicrons is from 0.04 to 2.5 and contacting the resulting oil with sufficient amount of an adsorbent to produce a transformer oil having a sludge-free Doble life of at least 64 hours.

8. Process according to claim 7 wherein said unrefined distillate boils mainly in the range of 600-l000F 9. Process according to claim 8 wherein said unrefined distillate boils mainly in the range of 650-750F.

10. Process according to claim 7 wherein the product of said contacting is an oil useful as a transformer oil and having a sludge-free Doble life of at least 100 hours I with a total acid number less than 0.1, said product oil at 330 millimicrons is no less than 1.0. 

1. PROCESS FOR PREPARING A HYDROCARBON FLUID OF IMPROVED OXIDATION STABILITY FROM A SEVERELY HYDROREFINED OIL HAVING A VISCOSITY FROM 30 TO 500 SUS AT 100*F AND AN ULTRAVIOLET ABSORPTIVITY OF LESS THAN 0.04 IN THE 335 MILLIMICRON REGION, SAID PROCESS COMPRISING ADDING TO THE SEVERELY HYDROREFINED OIL AN UNREFINED DISTILLATE OF A RECYCLE STOCK OBTAINED FROM CATALYTIC CRACKING OF GAS OIL WHICH IS RICH IN POLYNUCLEAR HYDROCARBONS HAVING AT LEAST FOUR CONDENSED AROMATIC RINGS AND RECOVERING A BLENDED OIL HAVING AN ABSORPTIVITY AT 335 MILLIMICRONS AT LEAST 15% GREATER THAN THAT OF SAID HYDROREFINED OIL AND LESS THAN 2.5.
 2. Process according to claim 1 wherein said hydrorefined oil has a sludge-free Doble life of less than 48 hours and the resulting blended oil has a 335 UVA in the range of 0.4-2.5 and is contacted with sufficient amount of an adsorbent to produce a hydrocarbon oil, useful as an electrical insulator oil, and having a sludge-free Doble life of at least 64 hours.
 3. Process according to claim 2 wherein said hydrocarbon oil product is useful as a transformer oil and has a sludge-free Doble life of at least 100 hours with a total acid number less than .1, said oil having a viscosity at 100*F. in the range of 45-70 SUS, an ultraviolet absorptivity at 335 millimicrons in the range of 0.06-2.0, a sulfur content in the range of 25-800 p.p.m., and containing less than 25 p.p.m. of total nitrogen and less than 10 p.p.m. of basic nitrogen, the chemical composition of the hydrocarbons in said oil being such that the ratio of the ultraviolet absorptivity at 335 millimicrons to the absorptivity at 330 millimicrons is no less than 1.0.
 4. A process according to claim 3 wherein said hydrocarbon oil has a sludge-free Doble life of at least 200 hours at a total acid number less than 0.01 and wherein said absorptivity at 335 millimicrons is in the range of 0.3 to 1.0.
 5. Process according to claim 1 wherein the ultraviOlet absorptivity of said severely hydrorefined oil is determined for at least one wave length or band in the 320-340 millimicron region.
 6. Process according to claim 5 wherein said wave length or band is at about 335 millimicrons.
 7. Process for preparing a hydrocarbon fluid useful as an electrical insulation medium from a severely hydrorefined naphthenic oil having a sludge-free Doble life less than 48 hours and a viscosity from 40 to 200 SUS at 100*F and having an ultraviolet absorptivity of less than 0.04 in the 335 millimicron region, said process comprising adding to said hydrorefined oil an unrefined distillate of a recycle stock obtained from catalytic cracking of gas oil and, which is rich in polynuclear hydrocarbons having at least four condensed aromatic rings, until said abosrptivity at 335 millimicrons is from 0.04 to 2.5 and contacting the resulting oil with sufficient amount of an adsorbent to produce a transformer oil having a sludge-free Doble life of at least 64 hours.
 8. Process according to claim 7 wherein said unrefined distillate boils mainly in the range of 600*-1000*F
 9. Process according to claim 8 wherein said unrefined distillate boils mainly in the range of 650*-750*F.
 10. Process according to claim 7 wherein the product of said contacting is an oil useful as a transformer oil and having a sludge-free Doble life of at least 100 hours with a total acid number less than 0.1, said product oil having a viscosity at 100*F. in the range of 50 to 65 SUS, an ultraviolet absorptivity at 335 millimicrons in the range of 0.06 to 2.0, a sulfur content in the range of 25 to 800 p.p.m. and containing less than 25 p.p.m. of nitrogen, the chemical composition of the hydrocarbons in said oil being such that the ratio of the ultraviolet absorptivity at 335 millimicrons to the absorptivity at 330 millimicrons is no less than 1.0. 